An infinitely variable speed transmission (traction drive) fluid has similar responsibilities to normal (geared) transmission (i.e. automatic transmission) fluids in that it must serve as a lubricant, coolant, and in some cases, hydraulic fluid. A traction or infinitely variable transmission (IVT) fluid has the added responsibility of transmitting torque from an input devise to the output through the lubricating film that it forms in the contact(s) between smooth rolling-sliding rotating elements of the transmission. Thus the fluid is required to exhibit high shear strength in the high shear stress EHD conditions found in the area of contact between the rolling-sliding drive elements which are separated and lubricated by a thin film of the IVT fluid. The fluid's resistance to shear (shear strength) in the contact provides the torque transmitting capability of the fluid composition.
Lubricating fluids suitable for use in infinitely variable transmissions for most equipment employed in outdoor applications need a critical balance of good low temperature flow properties and high EHD shear strength; and, particularly shear strength at a combination of high temperatures and low contact stresses. Cycloaliphatic hydrocarbons generally have superior shear strength properties but very poor low temperature properties compared to more typical hydrocarbon-based fluids employed for producing good lubricants. Other fluids with good low temperature properties can be incorporated which improve the low temperature properties of the mixture but generally these cause significant undesirable loss of the shear strength properties. It has now been found that simple low-viscosity dimethylsilicone fluids can accomplish the desired balance of these properties.
One of the bases of this concept, improvement of low temperature properties of cycloaliphatic fluids with silicone fluids is not new. Many types of aliphatic, cycloaliphatic and aromatic containing silicone fluids have been reported for this purpose and specifically for traction fluids. However, no report has been found that introduces just simple dimethylsilicone fluids as low temperature improvers. One reason may be the generally poor hydrocarbon solubility of most dimethylisilicone fluids which could be construed as limiting their utility. Therefore, other inventors have focused on silicone fluids containing other functional besides mainly methyls in an effort to avoid the perceived incompatibility issue; or, have resorted to incorporation a third component fluid in traction fluid formulations, such as aromatic hydrocarbons or esters in the mixture to induce the compatibility of the dimethylsilicone fluid in the hydrocarbon fluid.
These traction fluids which consist essentially of cycloaliphatic hydrocarbon in combination with the dimethylsilicone fluids serve as base oils to which additional additives may be added to form fully-formulated automatic transmission fluids for infinitely variable transmission fluids. These additives include antioxidant agents, antiwear agents, extreme pressure agents, detergents, dispersants, antifoamer, anti-rust agents, friction modifiers, and viscosity modifier additives. The silicone oil for the purposes of the instant invention used may consist essentially of up to but less than 10% other appropriate functional groups, longer-chain aliphatic, cycloalipahatic or aromatic, or combinations of these functionalities besides methyls to further enhance performance and the performance of the fully-blended traction drive or infinitely variable transmission fluid.